The present invention relates generally to the field of the placement of forensic tracking information on an optical disc and more particularly to the use of the limitations of the control and display data p-bit register in the lead-out zone to place hidden data on the optical disc.
Optical discs have become an increasingly popular and cost-effective means for digitally storing data. A typical optical disc comprises a circular disc having a recording layer of light reflective material embedded in a refractive substrate. The recording layer is disposed along a plane substantially normal to an axis about which the disc is rotated and stores data in the form of localized pits and lands along a continuously extending spiral track. The length of the pits and lands corresponds to one of the selected number of data symbols (for example, from 3T to 11T, with T of a determined length).
The spiral track has a lead-in zone which allows the optical reading device to begin reading the user data from the disc. Following the lead-in zone is an area, usually the majority of the disc space, where the user data is written thereto. The track concludes with a lead-out zone which notifies the reading device that all the user data has been read.
The data symbols on the optical disc are recovered through the use of a light source (such as a laser) which applies light of selected wavelength to the rotating disc and a transducer which generates a readback signal indicative of the data in relation to the relative differences in reflectivity of the pits and lands. It is common to separate the relative elevations of the pits and lands by a distance equal to a quarter-wavelength of the applied light so as to facilitate a substantial change in the amount of light reflected by the pits as compared to the amount of light reflected by the lands.
One popular optical format is commonly referred to as a compact disc, or CD, which has found widespread use in recent years in computer applications (such as CD-ROM) and in the music recording industry (audio CD). Another popular optical disc format is commonly referred to as digital versatile disc, or DVD.
Due to worldwide consumer demand for the types of information available on optical discs (e.g. software and music), combined with the relative ease with which unauthorized copies of optical discs can be generated, attempts have been made to implement various copy protection schemes to restrict unauthorized replication of the discs. One type of copy protection involves configuring an optical disc such that an authorized copy will function properly in a readback machine, but an unauthorized copy is prevented from being properly read.
Another type of copy protection is the placement of hidden data or induced error on the disc, commonly known as forensic tracking. During the creation of the optical disc, information is embedded in the disc through various techniques. A common type of forensic tracking provides that if the optical disc is illegally copied, the tracking information will be copied along with the user data. Therefore, forensic tracking does not prevent unauthorized replication, but places xe2x80x9cmarkingsxe2x80x9d on the replicated disc, whereby these markings may be used to determine the authenticity of the disc.
Another usage of forensic tracking is to hide proprietary information on the disc. This information is not used for verification purposes, but may be used to determine ownership and other rights, should an unauthorized copy be found.
A CD has an outer diameter of 120 millimeters (4.724 inches) and a data storage capacity of about 650 megabytes (MB). On a standard CD, 588 channel pits compose 1 frame. This frame, a raw data frame, is comprised essentially of a sync block, a control display symbol block, two Data blocks and two parity blocks.
The control display block is further composed of an 8-bit subcode block. The blocks are designated as P, Q, R, S, T, U, V and W,. commonly referred to as the PQ codes. The CD player collects a single set of subcode symbols from ninety-eight consecutive frames to form a subcode block with eight 98-bit words. Only the P and Q bits are used in audio CDs, whereas the other bits are available for encoding other information.
The p-bits were designed to be used as a simple pointer to the pauses in between audio tracks, the idea being that inexpensive CD audio players could xe2x80x9cskipxe2x80x9d to the next pause point instead of having to fully decode the Q channel which contains specific information such as the total number of selections on the disc, their beginning and ending points and timings, and lead-in and lead-out point. Most modern CD readers no longer use the p-bit information for seeking purposes.
The p-bit also has been designated to toggle on and off at a rate of two (2) Hz beginning two seconds after the lead-out zone has been reached and continuing until the end of the disc. Due to the physical limitations of the optical disc, combined with the two (2) Hz toggle rate, a problem exists in having the p-bit toggle properly. The optical disc reader reads seventy-five (75) sectors per second. The p-bit must make four state changes, such as ON to OFF, OFF to ON, ON to OFF and OFF to ON, within this time. As four states changes cannot be evenly divided between seventy-five sectors, where a sector is a full raw data frame having one subcode per frame, a perfectly symmetrical 2 Hz toggle cannot be achieved.
Several solutions have been implemented to provide this 2 Hz toggling of the p-bit register. One possible solution is inserting one state having one less sector than the other three states. For instance, it is possible to write the first 18 consecutive sectors of the p-bit in the ON state, write the following 19 sectors in the OFF state, the following 19 sectors in the ON state and the final 19 sectors in the OFF state. This gives an 18/19/19/19 sector toggle pattern. It is also possible to place the 18 sector state at any other point, provided the other states maintain 19 sectors. Thus, the seventy-five sectors are used to provide the proper 2 Hz toggle of the p-bit register.
Another possible solution is writing every state as having a total of 19 sectors before transition. Therefore, a sector toggle pattern would be 19/19/19/19. This solution produces a p-bit signal that toggles slightly over 2 Hz.
The 2 Hz toggle of p-bit register in the lead-out zone is a CD standard, thus all CDs must have the p-bit properly toggle within the lead-out zone. As the p-bit fails to contain any user information, only switching from ON to OFF or OFF to ON, this location is often overlooked as containing any forensic tracking information. It is to the placement of forensic tracking information using the p-bit register toggling requirement in the lead-out zone of an optical disc that the present invention is directed.
The present invention provides for the placement of forensic tracking information on an optical disc without compromising the integrity of the user data or the disc itself. The physical limitations of the disc, as they relate to standardization requirements, provide the requisite medium for encoding hidden forensic data. An optical disc of the present invention contains forensic tracking information in its lead-out zone created by the pattern of oscillating transition states used to create an oscillating signal at a specified frequency.
The preferred embodiment of the present invention provides for the placement of a forensic tracking message in a data string. The contents of the data string are encoded on the lead-out zone of the optical disc by p-bit toggle patterns that produce a nominal 2 Hz square wave.
As the 2 Hz p-bit lead-out toggle may be created by placing an 18 sector state amongst three 19 sector states, the location of the 18 sector state may be used for forensic purposes. The four possible sector arrangements, (18/19/19/19, 19/18/19/19, 19/19/18/19, 19/19/19/18), may be used to represent an encoded forensic message. In this embodiment, there are two groupings through which the hidden data may be encoded. Each grouping provides for the representation of a first value, such as xe2x80x9c0xe2x80x9d and a second value xe2x80x9c1xe2x80x9d which may be used to create a binary representation of the hidden data. The first grouping provides for the 18 sector state to always be located in an odd numbered position, either the first or third position, in the sector state sequence. The second grouping provides for the 18 sector state to always be located in the even numbered position, either the second or fourth position, in the sector state sequence. Thus, an unauthorized user will be less likely to detect the encoded data within the p-bit toggle as the toggling pattern will be more standardized.
The preferred embodiment further provides for the placement of error correction codes in the encoded message. The error correction codes are inserted prior to modulation and provide greater accuracy in the subsequent decoding of the hidden data. Also, the preferred embodiment provides for the writing of a begin-synchronization pattern and an end-synchronization pattern in the p-bit registers before and after the hidden data, respectively. Therefore, a hidden data extraction device may better determine the location of the forensic tracking information and insure a more accurate detection of the encoded data string.
Another embodiment of the present invention also provides for the placement of a forensic tracking message on the optical disc. The forensic tracking information is encoded on the lead-out zone of the optical disc by four distinct toggle patterns. The encoded data string itself is composed of four different values, such as xe2x80x9c00xe2x80x9d, xe2x80x9c01xe2x80x9d, xe2x80x9c10xe2x80x9d and xe2x80x9c11xe2x80x9d. These values are represented by different p-bit toggle patterns designed by the location of the 18 sector state with respect to the 19 sector states.
Also, the present invention provides for the transferring of the forensic tracking information through the creation of an optical disc having the hidden data encoded in the lead-out zone during the mastering process. A special encoder is used, in conjunction with a modulator, to provide a write signal to a disc mastering system having the forensic tracking information encoded and modulated therein. A disc master is produced in accordance with normal mastering procedures. All replicated discs thus contain the encoded forensic tracking information in the lead-out zone where the forensic information does not interfere with the normal operation of the optical disc.
A second aspect of the transferring of forensic tracking information with an optical disc created in accordance with the present invention is the extraction of the hidden data. The optical disc reader is directed to read the lead-out zone. A hidden data extraction module is used to specifically look for the forensic tracking information. In the preferred embodiment, the module will locate the begin-synchronization pattern and begin extracting the data string.
The module reads the p-bit toggle pattern, noting the location of the 18 sector state amongst the 19 sector states to determine the corresponding bit pattern of the encoded message. Once the end synchronization pattern is found, the module discontinues reading the p-bit toggle pattern, unless another begin synchronization pattern is found. The p-bit toggle pattern is used to represent the encoded hidden data, which is then decoded, providing the forensic tracking information.
Thus, without knowledge of the hidden data encoding scheme, a normal user may be able to detect the patterns in the p-bit 2 Hz toggle, but will unable to read the forensic tracking information. Furthermore, the hidden data is located within a standard toggle pattern thus the integrity and data content of the optical disc is not compromised.